Etching process for micromachining crystalline materials and devices fabricated thereby

ABSTRACT

The present invention provides an optical microbench having intersecting structures etched into a substrate. In particular, microbenches in accordance with the present invention include structures having a planar surfaces formed along selected crystallographic planes of a single crystal substrate. Two of the structures provided are an etch-stop pit and an anisotropically etched feature disposed adjacent the etch-stop pit. At the point of intersection between the etch-stop pit and the anisotropically etched feature the orientation of the crystallographic planes is maintained. The present invention also provides a method for micromachining a substrate to form an optical microbench. The method comprises the steps of forming an etch-stop pit and forming an anisotropically etched feature adjacent the etch-stop pit. The method may also comprise coating the surfaces of the etch-stop pit with an etch-stop layer.

RELATED APPLICATIONS

[0001] This application is a continuation-in-part of Application No.10/071,261 filed on Feb. 7, 2002, which in turn claims the benefit ofpriority of U.S. Provisional Application No. 60/266,931, filed on Feb.7, 2001, the entire contents of which are incorporated herein byreference. Applicants claim the benefit of priority of U.S. ProvisionalApplication No. 60/306,568, filed on Jul. 19, 2001, the entire contentsof which are incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates generally to devices havingintersecting structures etched into a substrate and a method for makingsuch devices, and more specifically to structures having a planarsurface formed along a selected crystallographic plane of a singlecrystal substrate, where the method provides that such a surface retainsa selected planar orientation at the point of intersection of such asurface with surfaces of additional structures in the substrate so that,for example, perfect convex corners are maintained.

BACKGROUND OF THE INVENTION

[0003] The ability to precisely locate optical elements relative to oneanother is of critical importance in the fabrication of micro-opticaldevices, since the alignment tolerances between elements are oftenspecified in submicron dimensions. Typically, such elements may includean optical signal source, such as a laser, a detector, and an integratedor discrete waveguide, such as a fiber-optic or GRIN rod lens.Additionally, such elements may include a fiber amplifier, opticalfilter, modulator, grating, ball lens, or other components for conveyingor modifying an optical beam. Micro-optical devices containing suchcomponents are crucial in existing applications such as opticalcommunication and consumer opto-electronics, as well as applicationscurrently being developed, such as optical computing.

[0004] Maintaining precise alignment among the optical elements may beconveniently provided by an optical microbench, such as a siliconoptical bench. An optical microbench comprises three-dimensionalstructures having precisely defined surfaces onto which optical elementsmay be precisely positioned. One material well-suited for use as anoptical microbench is single crystal silicon, because single crystalsilicon may be etched anisotropically to yield three-dimensionalstructures having planar sidewalls formed by the precisely definedcrystallographic planes of the silicon. For example, the {111 } siliconplane is known to etch more slowly than the {100} or {110} planes withproper choice of etchant. Thus, structures may be formed comprisingwalls that are {111 } planes by anisotropic etching.

[0005] Since the optical elements sit within the three-dimensionalstructures at a position below a top surface of the silicon substrate, aportion of the optical path often lies below the top surface of thesubstrate, within the volume of the substrate. Accordingly, passagewaysmust be provided in the optical microbench between three-dimensionalstructures so that light may travel between the elements disposed in theassociated three-dimensional structures. Hence, an optical microbenchshould contain three-dimensional structures that communicate with oneanother through structures such as a passageway.

[0006] While discrete, non-communicating, three-dimensional structuresmay be conveniently formed by an isotropic etching, etched structureswhich communicate with one another at particular geometries, such as aconvex corner, pose significant problems for applications in which it isdesirable to maintain the precise geometry defined by thecrystallographic planes. For example, where two {111 } planes intersectat a convex corner, the convex comer does not take the form of astraight line intersection between two planes, but rather rounds tocreate a rounded intersection between the two {111 } planes. As etchingcontinues to reach desired depth of the structure containing the {111 }planes, the rounding of the corners can grow to such an extent that asubstantial portion of the intersection between the two {111 } planes isobliterated. Since the {111 } planes are provided in thethree-dimensional structures to form a planar surfaces against whichoptical elements may be precisely positioned, absence of a substantialportion of the {111 } planes at the intersection can introduce a greatdeal of variability of the positioning of the elements at theintersection. Thus, the benefits provided by the crystallographic planescan be unacceptably diminished.

[0007] Traditionally, to avoid etching intersecting features, dicing sawcuts may be used. However, dicing saw cuts can be undesirable, becausesuch cuts typically must extend across the entire substrate and may notconveniently be located at discrete locations within the substrate.Moreover, dicing saw cuts create debris which may be deposited acrossthe substrate surface and lodge within the three-dimensional structures,which may interfere with the precise positioning of optical elementswithin such a structure.

[0008] Therefore, there remains a need in the art for optical microbenchtechnology which permits three-dimensional structures havingcrystallographic planar surfaces to intersect with other surfaces,without degrading the crystallographic orientation of the intersectedplanar surfaces.

SUMMARY

[0009] The present invention provides an optical microbench comprising asubstrate having an etch-stop pit and an etched feature, such as ananisotropically etched feature, disposed adjacent the etch-stop pit. Theanisotropically etched feature may comprise a V-groove. The etch-stoppit may have a shape suited to supporting an etch-stop layer on thesurfaces of the etch-stop pit. The etch-stop pit may be created prior tocreating the etched feature. The etch-stop layer comprises a materialresistant to the etchant which is used to create the etched feature.After the etch-stop layer is provided, the etched feature is etched inthe substrate. The etch-stop layer prevents the feature etching fromextending into the region where the etch-stop pit is located. Theprevention of such etching by the etch-stop layer provides that thecrystallographic planner walls of the anisotropically etched featuremaintain their crystallographic orientation adjacent the stop-etch pit.

[0010] The present invention also provides a method for micromachining asubstrate to form an optical microbench. The method comprises the stepsof forming an etch-stop pit and forming an anisotropically etchedfeature adjacent the etch-stop pit. The method may also comprise coatingthe surfaces of the etch-stop pit with an etch-stop layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The foregoing summary and the following detailed description ofthe preferred embodiments of the present invention will be bestunderstood when read in conjunction with the appended drawings, inwhich:

[0012]FIG. 1 schematically illustrates a top view of a V-groove providedin an upper surface of a substrate, where the V-groove includes two endswhich include wedge-shaped end portions;

[0013] FIGS. 2A-2D schematically illustrate top views of a substrateshowing the changes to the substrate as features are added to thesubstrate to create a structure having a V-groove and an adjoiningetch-stop pit in accordance with a first embodiment of the presentinvention;

[0014] FIGS. 3A-3F and 4A-4D schematically illustrate top views ofalternative etch-stop pit configurations with the adjoining V-grooves inaccordance with the present invention;

[0015]FIG. 5 schematically illustrates a top view of an alternativeetch-stop pit configuration in accordance with the present invention forpreventing formation of a wedge-shaped end wall in a V-groove;

[0016]FIG. 6 schematically illustrates a top view of an alternativeetch-stop pit configuration in accordance with the present invention forproviding a partial, wedge-shaped end wall in a V-groove;

[0017]FIG. 7 schematically illustrates a top view of an alternativeetch-stop pit configuration in accordance with the present invention forpreventing formation of a wedge-shaped end walls in three V-grooveswhich adjoin the etch-stop pit;

[0018] FIGS. 8-10 schematically illustrate top views of furtherconfigurations of etch-stop pits and V-grooves along with a device mountfor providing optical subassemblies in accordance with the presentinvention;

[0019] FIGS. 11-16 schematically illustrate top views of alternativeconfigurations etch-stop pits with two or more V-grooves adjoining theetch-stop pits;

[0020] FIGS. 17-20 schematically illustrate top views of substrateshaving etch-stop pits, V-grooves, and an optional V-pit, for providingoptical subassemblies in accordance with the present invention;

[0021] FIGS. 21-26 schematically illustrate top views of substrateshaving two V-grooves oriented at 90 degrees with respect to one anotherand having an etch-stop pit disposed at the location of the intersectionof the two V-grooves;

[0022]FIGS. 27 and 28 schematically illustrate top views of substrateshaving an etch-stop pits disposed at locations where an inside, convexcorner of two intersecting V-grooves would be located;

[0023] FIGS. 29A-29D, 30, 31, 32A-32B, and 33 schematically illustratetop views of substrates having an etch-stop pit which circumscribes aselected area of the substrate in which an anisotropically etchedfeature is formed;

[0024]FIGS. 34, 35A, 36, and 37 schematically illustrate top views ofsubstrates having a U-shaped etch-stop pit adjacent to a V-pit toprovide a location on the substrate for a laser mount and to provide alocation for retaining a spherical optical element;

[0025]FIG. 35B schematically illustrates a cross-sectional view of thesubstrate illustrated in FIG. 35A;

[0026]FIGS. 38A and 39A schematically illustrate top views of substrateshaving a V-groove with an etch-stop pit and fiber stop disposedinternally to the groove;

[0027]FIGS. 38B and 39B schematically illustrate cross-sectional viewsof the substrates illustrated in FIGS. 38A and 39A, respectively;

[0028]FIG. 40 illustrates a flowchart representing a process inaccordance with the present invention for creating an etch-stop pit andan adjacent anisotropically etched feature;

[0029]FIG. 41 illustrates a flowchart representing another process ofthe present invention for creating an etch-stop pit and adjacent ananisotropically etched feature;

[0030]FIG. 42 illustrates a flowchart representing yet another processof the present invention for creating an etch-stop pit and an adjacentanisotropic feature;

[0031]FIGS. 43 and 44 schematically illustrate a top view and across-sectional view, respectively, of a substrate comprising a V-grooveand an adjoining etch-stop pit;

[0032] FIGS. 45-51 schematically illustrates cross-sectional views of asubstrate at selected steps of processing in accordance with the methodillustrated in the flowchart of FIG. 41;

[0033] FIGS. 52-58 schematically illustrates cross-sectional views of asubstrate at selected steps of processing in accordance with the methodillustrated in the flowchart of FIG. 42; and

[0034] FIGS. 59-64 schematically illustrates cross-sectional views of asubstrate at selected steps of processing in accordance with the methodillustrated in the flowchart of FIG. 43.

DETAILED DESCRIPTION OF THE INVENTION

[0035] Referring now to the figures, wherein like elements are numberedalike throughout, several different embodiments of devices in accordancewith the present invention are illustrated. The different embodimentsinclude a substrate having at least two common features, an etch-stoppit and an anisotropically etched feature adjacent the etch-stop pit.The etch-stop pit has a shape suited to supporting an etch-stop layer onthe surfaces of the etch-stop pit. The etch-stop pit is created prior tocreating the anisotropically etched feature. The etch-stop layercomprises a material resistant to the etchant which is used to createthe anisotropically etched feature. After the etch-stop layer isprovided, the anisotropically etched feature is etched in the substrate.The etch-stop layer prevents the anisotropic etching from extending intothe region where the etch-stop pit is located. The prevention of suchanisotropic etching by the etch-stop layer provides that thecrystallographic planner walls of the anisotropically etched featuremaintain their crystallographic orientation adjacent the stop-etch pit.The advantages of preventing such etching are illustrated in theaccompanying figures depicting several desirable embodiments of thepresent invention.

[0036] Throughout the figures, the substrate material is selected to be<100>-oriented silicon. However, other orientations of silicon, such as<110>-oriented silicon, may also be used in accordance with the presentinvention. In addition, other anisotropic crystalline materials, such asIII-V semiconductor materials, e.g., InP, GaAs, InAs, or GaP, may beused in accordance with the present invention. The substrate material ischosen with regard to the nature of the particular optical device andthe features to be fabricated. The crystal orientation of the substratemay be chosen with respect to the desired orientation of the sidewallsof the fabricated features. For example, <100>-oriented silicon may beselected to create a features having sidewalls that are sloped withrespect to the upper surface of the substrate. Alternatively,<110>-oriented silicon may be selected to create features havingsidewalls that are perpendicular to the upper surface of the substrate.

[0037] An example of a typical feature which may be formed byanisotropic etching in an <100>-oriented silicon substrate 6 is aV-groove 2, as illustrated in FIG. 1. In a first aspect of the presentinvention a modified V-groove 2 is provided, V-groove 12, having aconfiguration particularly well-suited to retaining a cylindricalelement, such as an optical fiber or GRIN rod lens, as illustrated inFIGS. 2A-2D.

[0038] Turning first to the V-groove 2 illustrated in FIG. 1, eachsurface of the V-groove 2 is a {111 }-plane of the silicon substrate 6.The V-groove 2 may be made by known methods such as etching through arectangular aperture mask using an aqueous solution of KOH. A V-groove 2which does not extend to the edges of the substrate 6 includes twowedge-shaped end walls 4. The end walls 4 slope upwardly towards theupper surface 1 of the substrate 6 from an apex 5. A wedge-shaped endwall 4 is often undesirable in optical subassemblies, because awedge-shaped end wall 4 can partially or completely occlude the opticalpath to block light transmitted to or from an optical element disposedin the V-groove 2. In addition, the wedge-shaped end wall 4 functionspoorly as an optical fiber stop, since the wedge-shaped end wall 4 issloped with respect to the endface of the fiber which is usuallyperpendicular to the longitudinal axis of the fiber. Thus, it isdesirable to create a V-groove without one or more of the wedge-shapedend walls 4.

[0039] In particular, referring to FIG. 2D, a V-groove structure inaccordance with the present invention is illustrated where one of thewedge-shaped end walls 14 a, shown in phantom, is eliminated from theV-groove 12. The device includes a substrate 10 having an upper surface11 in which a V-groove 12 and adjacent etch-stop pit 16 are provided.The edges 13 where the V-groove 12 and the stop-etch pit 16 intersectare straight line segments that lie within the {111 }-plane of theV-groove sidewalls 15. The ability to remove the right end wall 14 awhile maintaining the {111 }-orientation of the sidewalls 15 in thevicinity of the removed end wall 14 a is provided by the etch-stop pit16 and etch-stop layer 18.

[0040] The sequence in which the etch-stop pit 16 and V-groove 12 areformed in the surface of the substrate 10 is illustrated in FIGS. 2A-2D.Turning to FIG. 2A, a top elevational view of the substrate 10 is shownin which an etch-stop pit 16 is formed. As depicted, the etch-stop pit16 has a triangular cross-section in the plane of the upper surface 11of the substrate 10. Other shapes than triangular cross-section may beused so long as such shapes are suited to preventing the formation of awedge-shaped end wall 14 a of the V-groove 12. The types of shapes whichmay be used are discussed below with reference to FIGS. 3 and 4.

[0041] The walls of the etch-stop pit 16 may desirably extend into thesubstrate 10 at a 90 degree angle, i.e. vertical, relative to the uppersurface 11 of the substrate 10, and the etch-stop pit 16 may contain aflat bottom parallel to the plane of the upper surface 11. Such aconfiguration of the etch-stop pit 16 may be fabricated by high-aspectratio dry etching, such as reactive ion etching. Alternatively, theetch-stop pit 16 may include sidewalls that are sloped with respect tothe plane of the upper surface 11. Regardless of the sidewall slope thatis utilized, the portions of the sidewalls 16 a located proximate theregion at which the V-groove 12 is to be formed, i.e. at intersectingsegments 13, should extend into the substrate 10 a greater depth thanthe depth intended for the adjoining portion of the V-groove 12.Providing such a deeper sidewall portion ensures that a subsequentlyapplied etch-stop layer 18 provides a barrier between an etchant in theV-groove 12 and the etch-stop pit 16.

[0042] After formation of the etch-stop pit 16, an etch-stop layer 18 isconformally provided on the sidewalls 16 a and the bottom of theetch-stop pit 16. The etch-stop layer 18 comprises a material that isresistant to the etchant that will be used to create the V-groove 12.For example, the etch-stop layer 18 may comprise silicon dioxide, whichmay be provided by CVD or by thermally oxidizing surfaces 16 a of theetch-stop pit 16, or silicon nitride, which may be provided by CVD.Optionally, the upper surface 11 of the substrate 10 may be providedwith a layer of the same material used for the etch-stop layer 18.

[0043] The V-groove 12 is formed in the surface 11 of the substrate 10by a suitable process, such as anisotropic wet etching with KOH or EDPthrough a rectangular aperture mask. The rectangular aperture mask isoriented such that the perimeter of the rectangular aperture isregistered to the perimeter of the V-groove 12 located in the uppersurface 11 of the substrate 10. The rectangular aperture mask isoriented such that a portion of an end of the rectangular apertureoverlies the etch-stop pit 16. Further details regarding how the masksare provided is discussed below in connection with the method of thepresent invention.

[0044] As an optional additional step, the etch-stop layer 18 may beremoved from the etch-stop pit 16. Removal of the etch-stop layer allowsthe V-groove 12 to communicate with the etch-stop pit 16. Suchcommunication permits an element, such as a fiber, disposed within theV-groove 12 to extend into the region over the etch-stop pit 16 and abutthe sidewall 16 a of the etch-stop pit 16 that is disposed perpendicularto the longitudinal axis of the V-groove 12, to provide a fiber stop 17.

[0045] The process described above with respect to FIG. 2D is suited toforming all of the structures illustrated herein. For example, each ofthe following structures described below includes at least one etch-stoppit which is formed before an adjacent anisotropically etched feature,such as a V-groove is formed adjacent to the pit. In addition, anetch-stop layer is provided in the etch-stop pit prior to forming aanisotropically etched feature. While the etch-stop layer may not beillustrated in the figures, because it has been removed after theformation of the anisotropically etched feature, it is understood thatthe etch-stop layer is present within the etch-stop pit while theanisotropically etched feature is being formed.

[0046] In addition to the triangular cross-sectional shape of theetch-stop pit 16 illustrated in FIGS. 2A-2D, other cross-sectionalshapes may be used in accordance with the method of the presentinvention to completely or partially prevent the formation of awedge-shaped end wall of a V-groove, as shown in FIGS. 3C-3F and 4A-4D.For example, a first type of etch-stop pit configuration for completelypreventing the formation of a wedge-shaped end wall 44 is illustrated inFIGS. 4A-4D. FIGS. 4A-4D illustrate top elevational views of a V-groove42 adjacent to etch-stop pits 46, 47, 48, 49 of differingcross-sectional areas. In each configuration, the etch-stop pit 46, 47,48, 49 completely overlays a region of the substrate in which thewedge-shaped end wall 44 of the V-groove 42 would otherwise be formed.The etch-stop pit 46, 47, 48, 49 and V-groove 42 may be formed in thesubstrate by the procedure described above with respect to the deviceillustrated in FIG. 2D.

[0047] One desirable configuration of an etch-stop pit 49 comprises twosidewalls joined at an apex that lies along the longitudinal axis of theV-groove 42 such that the apex angle, α, is bisected by the longitudinalaxis. Such a configuration of an etch-stop pit 49 can prevent theformation of a wedge-shaped end wall 44 when the apex angle is less thanor equal to 90 degrees.

[0048] If the apex angle were greater than 90 degrees, as illustrated inFIGS. 3C and 3D, a partial wedge-shaped end wall 34 would be formed inthe V-groove 32. In the configuration where the “apex angle” is equal to180 degrees, i.e. a straight line, the typical wedge-shaped end wall 24would be formed in the V-groove 22, as illustrated in FIGS. 3A and 3B.That is, an etch-stop pit 26 having a straight sidewall 23 adjacent tothe area in which the V-groove 22, is to be formed, and orientedperpendicular to the longitudinal axis of the V-groove 22, allows forthe formation of a wedge-shaped end wall 24. Other cross-sectionalshapes of an etch-stop pit are contemplated in accordance with thepresent invention, such as the “W” cross-sectional shape depicted inFIGS. 3E and 3F.

[0049] Yet another configuration of an etch-stop pit 386 in accordancewith the present invention may be provided so that a fiber stop 387 iscreated within a V-groove 384, as illustrated in FIGS. 38A-38B and39A-39B. FIGS. 38A and 39A illustrated top views of a substrate 380 inwhich a V-groove 384 is formed. FIGS. 38B and 39B illustratecross-sectional views taken along the lines B-B in FIGS. 38A and 39A,respectively. The etch-stop pit 386 has a shape that promotes theformation of a wedge-shaped fiber stop 387 along a {111 }crystallographic plane adjacent a first sidewall 383 of the etch-stoppit 386. In particular, the straight sidewall 383 oriented perpendicularto the longitudinal axis of the V-groove 384 promotes the formation ofthe wedge-shaped fiber stop 387 in an analogous fashion to the formationof the wedge-shaped end wall 24 in FIG. 3B. The etch-stop pit 386 alsocomprises a pair of angled sidewalls 385 across the dark and 386 fromthe first end wall 383. The angled sidewalls 385 intersect at a selectedapex angle which has a magnitude and orientation suitable for preventingthe formation of wedge-shaped surfaces, i.e. {111 } surfaces, in theV-groove 384 adjacent to the angled sidewalls 385. The angled sidewalls385 may have a similar configuration to corresponding sidewalls depictedin FIG. 2D. As illustrated in the cross-sectional views of FIGS. 38B and39B, the wedge-shaped fiber stop 387 extends above the deepest portionof the V-groove 384 so that a fiber 381 disposed within the V-groove 384may abut the wedge-shaped fiber stop 387.

[0050] A second type of etch-stop pit configuration that prevents awedge-shaped end wall from forming has a parallelogram cross-sectionalshape oriented at an angle, β, of 45 degrees or less, with thelongitudinal axis of the V-groove 52, as illustrated in FIG. 5. If, theangle, β, is larger than 45 degrees, as depicted in the configuration ofFIG. 6, then a partial wedge-shaped end wall 64 is formed in theV-groove 62. In a case where β is 90 degrees, the configuration of theetch-stop pit becomes functionally equivalent to that of the etch-stoppit depicted in FIG. 3A. In addition, V-grooves 52, 53, 55 may beprovided on opposing sides of the etch-stop pit 56 as illustrated inFIG. 7. So long as the longitudinal axis of each V-grooves 52, 53, 55 isoriented at an angle less than 45 degrees relative to an adjacentsurface of the etch-stop pit 56, the etching process in accordance withthe present invention will not produce wedge-shaped end walls in theV-grooves 52, 53, 55 in the region adjacent the etch-stop pit 56. Anynumber of V-grooves may be so provided, and such grooves need not havethe same size.

[0051] Returning now to the configuration illustrated in FIG. 2D, wherethe combined V-groove 14 and etch-stop pit 16 provide a cavity having afiber stop 17 for retaining a fiber optic, further optical subassembliesmay be fabricated by providing additional features in or on thesubstrate 10. Such subassemblies may provide for optical communicationwith the fiber. In particular, the structure of FIG. 2D is well-suitedfor use with other optical elements, because the fiber stop 17 providesa fiducial reference point to precisely identify where the end of thefiber is located.

[0052] For example, FIGS. 8-10 illustrate top elevational views ofadditional configurations in accordance with the present invention thatprovide optical subassemblies comprising a fiber 81, 91, 101, a V-groove84, 94 104, and a laser mount 85, 95, 105. Alternatively, detectormounts could be provided in place of the laser mounts 85, 95, 105. Inparticular, with reference to FIG. 8, a V-groove 84 and adjoiningetch-stop pit 86 with fiber stop 83 are provided in a configurationsimilar to that depicted in FIG. 2D described above. The etch-stop pit86, however, is not precisely triangular in cross-section, but ratherincludes an etched area 87 that protrudes, in cross-section, from thefiber-stop edge of the etch-stop pit 86, so that the cross-sectionalshape of the etch-stop pit 86 is similar to that of an arrowhead. Theetched area 87 allows for beam expansion. In addition, a laser mount 85is provided proximate the etched area 87 and is disposed along thelongitudinal axis of the V-groove 84. It may be desirable to provide anoptical device between the end of the fiber optic 81 and the laser mount85. Accordingly, the configurations illustrated in FIGS. 9 and 10provide slots 99, 109 for receiving optical elements. The slots 99, 109communicate with the respective etch-stop pits 96, 106 and may be formedat the same time as the etch-stop pits 96, 106. The slots 99, 109comprise vertical sidewalls, however, sloped sidewalls may also beprovided. The slot 109 of FIG. 10 conveniently has a cross-sectionalshape of a plano-convex lens, whereas the slots 99 is well-suited toreceiving flat optics.

[0053] In yet another etch-stop pit configuration in accordance with thepresent invention, the etch-stop pit may have a diamond-likecross-sectional shape which is suited to device configurations thatinclude two or more V-grooves disposed on opposing sides of theetch-stop pit, as illustrated in FIGS. 11-16. Referring to FIG. 11, asubstrate 110 is shown which includes an a diamond cross-sectionalshaped etch-stop pit 116 with two V-grooves 114, 115 disposed onopposing sides of the etch-stop pit 116. The V-grooves 114, 115 havelongitudinal axes are collinear and intercept at a respective vertex ofthe etch-stop pit 116. The region of intersection between each V-groove114, 115 with the respective portion of the etch-stop pit 116, has asimilar geometry to the intersection between the V-groove 14 andetch-stop pit 16 depicted in FIG. 2D. Thus, for the same reasons givenabove, no wedge-shaped end wall is formed in the V-grooves 114, 115 atthe locations adjacent the etch-stop pit 116. To allow the end faces ofrespective fibers disposed in two V-grooves 164, 165 to be space moreclosely together, the etch-stop pit 166 may comprise a diamond-likeshape that is compressed, as illustrated in FIG. 16.

[0054] In a similar manner, the etch-stop pit 136 may have across-sectional shape suited to having a single V-groove 134 on one sideof the etch-stop pit 136 and having two or more V-grooves 135, 137, 139,disposed at an opposing side of the etch-stop pit 136. In addition, theetch-stop pit 136 may have a cross-sectional shape suited to preventingthe formation of a wedge-shaped end wall in each V-groove 134, 135, 137,139 at the respective positions where the V-grooves 134, 135, 137, 139adjoin the etch-stop pit 136. A suitable shape for such an etch-stop pit136 as depicted in FIG. 13. The etch-stop pit 136 provides two fiberstops 137 for a fiber disposed in the V-groove 134. Yet additionalshapes of an etch-stop pit 126 may be provided for preventing theformation of wedge-shaped end walls in multiple V-grooves 124, 125, 126,127, as illustrated in FIG. 12. Wedge-shaped end walls do not form forthe reasons given above with regard to FIGS. 4D and 7, for example.

[0055] Still further, two of the ‘etch-stop pit with adjoiningV-groove’-structures illustrated in FIG. 2D may be provided in a singlesubstrate 140 in back-to-back coaxial relationship with a passageway 149extending between the two triangular sections of the etch-stop pit 146,as illustrated in FIG. 14. Such a configuration provides a fiber stop147 for each of the V-grooves 144, 145 so that the distance, D, betweenthe ends of two fibers located within the V-grooves 144,145 may beprecisely specified. In addition, the passageway 159 may be sufficientlylong so as to provide for insertion of an optical element between thetwo fiber stops 157. A slot 153, or other suitable shape, is provided toreceive such an optical element.

[0056] In yet another aspect of the present invention, two or more theabove-described ‘etch-stop pit with adjoining V-groove’ structures maybe provided in a substrate with a V-pit disposed therebetween, as shownin FIGS. 17, 18, and 20. A V-pit 179, 189, 209 may be formed byanisotropic etching by the same methods used to form V-grooves but usinga square aperture mask rather than a rectangular aperture mask. TheV-pit 179, 189, 209 may be anisotropically etched at the same time asthe grooves 174, 184, 204. The V-pit 179, 189, 209 should be etchedafter the etch-stop pit 176, 186, 206 and the etch-stop layer areprovided, in accordance with the process described above in reference toFIG. 2D. The V-pit 179, 189, 209 comprises for triangular-shaped,sidewalls that lie in the {111 } crystallographic planes to form afour-sided regular pyramid that extends into the substrate 170, 180,200. Like the V-grooves 174,184 the V-pits 179,189 should extend intothe substrate a depth less than the depth of the etch-stop pits 176, 186at the point of intersection between the V-pits 179, 189 and theetch-stop pits 176, 186, as illustrated in FIGS. 17 and 18. In aconfiguration where the V-pit 209 does not intercept the etch-stop pit206, the V-pit 209 depth does not need to be selected with regard to thedepth of the etch-stop pit 206. The V-pits 179, 189, 209 provide aconvenient shape for retaining a spherical optical element the V-pits179, 189, 209, such as a ball lens. The V-grooves 174, 184, 204 arepositioned so that an optical element disposed within the V-grooves 174,184, 204 can optically communicate with the optical element disposedwithin the respective V-pit 176, 186, 206. In alternative configuration,as illustrated in FIG. 19, the etch-stop pit 196 may contain a centralportion 195 configured to hold a spherical optical element. For example,the central portion may have a diamond-like shape. The central portion195 of the etch-stop pit 196 may serve the same function of retaining aspherical optical element as that of the V-pit 189.

[0057] In another aspect of the present invention, an etch-stop pit maybe provided at a location where two anisotropically etched featureswould intersect to form an inside, convex corner. A convex corner formedby the intersection of two {111 } planes does not form a straight lineintersection between two planes, but rather creates a roundedintersection between the two intersecting {111 } planes. The roundingcan propagate to remove material in the vicinity of the intersection,such that the well-defined {111 } planes can be etched away in thevicinity of the intersection to yield structures that are not {111 }planes. Thus, it would be desirable to prevent the formation of suchrounded corners.

[0058] FIGS. 21-28 illustrate several configurations of etch-stop pitsin accordance with the present invention which are suited to preventundesirable etching at an inside, convex corner. Each of the structuresin FIGS. 21-28 may desirably be formed by the process described abovewith reference to FIG. 2D, with an etch-stop pit and etch-stop layerprovided in the substrate prior to anisotropically etching theV-grooves. For example, referring to FIG. 21, a top elevational view ofthe substrate 210 is shown in which two V-grooves 214 are disposed. Thetwo V-grooves 214 are oriented with their respective longitudinal axesat 90 degrees relative to one another. An etch-stop pit 216 is providedat a selected location of the substrate 210 corresponding to thelocation at which the two V-grooves 214 would otherwise intersect.Providing the etch-stop pit 216 at the selected location preventsintersection of the V-grooves 214. The etch-stop pit 216 may be disposedat a 45 degree angle, β, so that wedge-shaped end walls are not formedin the V-grooves 214 adjacent the etch-stop 216. To provide for greaterease of alignment (lower alignment tolerances) between the etch-stop pit216 and the longitudinal axis of the V-grooves 214, an angle of lessthan 45 degrees may be preferable.

[0059] Alternative configurations of an etch-stop pit that preventetching of an inside, convex corner and wedge-shaped V-groove end wallsare illustrated in FIGS. 22-28. Each configuration depicted in FIGS.22-28 includes V-grooves 224, 234, 244, 254, 264 oriented at 90 degreeswith an intermediate etch-stop pit 226, 236, 246, 256, 266 in a similarconfiguration to that of FIG. 21. The etch-stop pit 226, 236, 246, 256,266 is located to prevent intersection of the V-grooves 224, 234, 244,254, 264. Each of the etch-stop pits 226, 236, 246, 256, 266 hasstraight wall segments disposed at an angle of 45 degrees or less withrespect to the longitudinal axis of an adjacent V-groove 224, 234, 244,254, 264. Referring to FIGS. 22 and 23, the etch-stop pit 226, 236 mayinclude an interior portion 225, 235 for retaining optical element suchas filters, lenses, micromechanical switches, for example. In addition,as illustrated in FIGS. 25 and 26, the etch-stop pit 256, 266 may have ashape configured to provide a fiber stop 257, 267 for fibers 251, 261disposed within the V-grooves 254, 264.

[0060] In accordance with the present invention, yet additionalconfigurations of etch-stop pits 276, 286 are provided which permit theintersection of two V-grooves 274, 284 while preventing the formation ofan inside, convex corner 275, 285, thus obviating the need for cornercompensation, as illustrated in FIGS. 27 and 28. For example, a topelevational view of a substrate 270, 280 is shown in which pairs ofV-grooves 274, 284 are provided in an upper surface of the substrate270, 280. Pairs of V-grooves 274, 284 intersect at ends of the V-grooves274, 284 at an angle of 90 degrees. An etch-stop pit 276, 286 isprovided at a selected location of the substrate 270, 280 correspondingto the location at which an inside corner 275, 285 of the intersectingV-grooves 274, 284 would otherwise be formed. Providing the etch-stoppit 276, 286 coated with an etch-stop layer at the selected locationprevents formation of the inside convex comer 275, 285.

[0061] In a further aspect of the present invention, an etch-stop pit296 may be provided as a continuous boundary that circumscribes a regionof the substrate 294 that is to be anisotropically etched. Providingsuch an etch-stop pit boundary permits the anisotropically etchedfeatures to be etched more deeply than otherwise possible. For example,referring to FIG. 30, a cross-sectional view of a substrate 300 is shownin which a recessed V-groove 304 is provided. The ability to form theV-groove 304 below the plane of the upper surface 301 is provided by thepresence of the etch-stop pit 306 (coated with an etch-stop layer) whichcircumscribes the region in which the V-groove 304 is formed. If theetch-stop pit 306 were not provided, the surfaces of the V-groove 304would extend upward to the upper surface 301 as indicated by the dashedline 307, and thus would not be recessed with respect to the uppersurface 301.

[0062] Turning now to FIGS. 29A-D, an L-shaped etch-stop pit 296 isprovided which circumscribes an L-shaped area in which ananisotropically etched feature may be formed. Providing the L-shapedetch-stop pit 296 permits the formation of {111 } sidewalls asillustrated in FIGS. 29A and 29B. Etching for a longer period of timepermits a deeper feature to be formed, as illustrated in FIGS. 29C and29D. Alternatively, other shapes than L-shaped may be utilized as anetch-stop pit. For example, the etch-stop pit 316 may have a T-shapedcross-section as illustrated in the top view of FIG. 31. Uponanisotropically etching the region 311 bounded by the T-shaped etch-stoppit 316, {111 } sidewalls may be formed as illustrated in FIGS. 32A and32B. In the vicinity of the cross-sectioning plane B-B, theanisotropically etched feature 324 may have a V-shaped cross-section, asillustrated in FIG. 32B. Yet further shapes may be utilized inaccordance with the present invention as an etch-stop pit whichcircumscribes an area to be anisotropically etched, such as theconfiguration depicted in FIG. 33.

[0063] In yet another aspect of the present invention, a U-shapedetch-stop pit 346 is provided adjacent to a V-pit 345 to provide alocation on a substrate 340 for mounting an optical element, such as alaser mount 355, and to provide a location for retaining an opticalelement, such as a spherical optical element 350, as illustrated inFIGS. 34-36. FIGS. 34 and 35A illustrate a top view of a substrate 340in which a U-shaped etch-stop pit 346 is provided adjacent a V-pit 345.The U-shaped etch-stop pit 346 includes sidewalls that extend a selecteddepth into the substrate 340. Optionally, the sidewalls of the U-shapedetch-stop pit 346 may be vertical, as illustrated in FIG. 34.Alternatively, the sidewalls of the U-shaped etch-stop pit 346 may beinclined relative to the upper surface 301 of the substrate 340. Thesidewalls of the U-shaped etch-stop pit 346 are conformally coated withan etch-stop layer, or, optionally, the etch-stop pit 346 is filledetch-stop layer material. In addition, the portion 343 of the substratesurface 301 interior to the etch-stop pit 346 may be provided with anetch-stop layer. As explained above with reference to the process ofFIGS. 2A-2D, the etch-stop layer comprises a material that is resistantto the etching used to form an anisotropically etched feature, such asV-pit 345.

[0064] After the desired etch-stop layer or layers are provided, theV-pit 345 may be formed by anisotropic etching by the same methods usedto form V-grooves but using a square aperture mask. Instead of using aperfectly square aperture mask, a generally square-aperture thatincludes a protrusion to protect substrate surface portions 343 interiorto the etch-stop pit 346 may be used. The V-pit 345 may beanisotropically etched at the same time as the optional V-groove 354.The V-pit 345 should extend into the substrate a depth less than thedepth of the etch-stop pit 346 at the point of intersection 353 betweenthe V-pit 345 and the etch-stop pit 346, as illustrated in FIG. 35B. Ina configuration where the V-pit 345 does not intercept the etch-stop pit346, the V-pit 345 depth does not need to be selected with regard to thedepth of the etch-stop pit 346.

[0065] The V-pit 345 provides a convenient shape for retaining aspherical optical element, such as a ball lens 350. The interior portion343 of the substrate surface 301 provides a convenient location at whicha laser 355, or other optical device, may be located for opticalcommunication with the ball lens 350. Providing the U-shaped etch-stoppit 346 permits a portion of the V-pit 345 adjacent the etch-stop the346 to be recessed below the surface 341 of the substrate 340. Such arecess permits the ball lens 350 to be positioned more closely to thelaser 355, as illustrated in FIG. 35B. In addition, a V-groove 354, 374may also be provided for optical communication between a fiber disposedwith the V-groove 354, 374 and the V-pit, as illustrated in FIGS. 36 and37. With respect to FIG. 36, the V-groove 354 may be fabricated in asimilar manner as the V-grooves 174 of FIG. 17, for example.Alternatively, as illustrated in FIG. 37, the etch-stop pit 376 maycircumscribe the region in which the V-pit 375 is formed. The etch-stoppit 376 comprises a U-shaped segment 366 to provide an analogousfunction to that of the U-shaped etched pit 346 in the configuration ofFIG. 35A. The etch-stop pit 376 also comprises a triangular-shapedsegment 378 to prevent formation of a wedge-shaped end wall in theV-groove 374 and to provide a fiber stop 377.

[0066] Methods of Fabrication

[0067] In accordance with the present invention, there are providedmethods for fabricating optical subassemblies having an etch-stop pitand an adjacent recessed area, such as an anisotropically etched area,for receiving an optical element. Three exemplary methods areillustrated in the flowcharts of FIGS. 40-42 and the accompanying sidecross-sectional views of FIGS. 45-64. The orientation of the sidecross-sectional views of FIGS. 45-64 is illustrated in FIGS. 43 and 44.

[0068] Referring to FIG. 43, a top elevational view is shown of asubstrate 440 in which a V-groove 444 and adjacent etch-stop pit 446 areprovided. The structure shown in FIG. 43 is similar to that shown inFIG. 2D, where one of the wedge-shaped end walls is eliminated from theV-groove 444. A cross-sectional view taken along the line B-B isillustrated in FIG. 44 to show a cross-section of the V-groove 444 at alocation where the V-groove 444 intersects the etch-stop pit 446. FIGS.45-64 illustrate cross-sectional views of substrates which are takenalong the same view direction, B-B, as the cross-sectional view in FIG.44. The exemplary part fabricated by each of the methods illustrated inthe flowcharts of FIGS. 40-42 has a final configuration similar to thatof the device shown in FIGS. 43 and 44.

[0069] Referring now to FIG. 40, there is shown a flowchart illustratinga method in accordance with the present invention for creating thedevice illustrated in FIGS. 43 and 44. As illustrated in FIG. 45, asubstrate 450 made from <100>-oriented Si is provided. The processing ofthe substrate 450 begins at step 4000 of FIG. 40 by providing aprotective layer 452 on a first surface of the substrate 450 to coverthat portion of the substrate 450 in which the etch-stop pit 516 is notto be provided. That is, the protective layer 452 includes an etch-stoppit aperture 451 through which a portion of the substrate 450 surface isaccessible for forming the etch-stop pit 516.

[0070] The protective layer 452 may be deposited over the entire surfaceof the substrate 450. Thereafter, portions of the protective layer 452may be removed to expose the surface of the substrate 450 at theselected area for the etch-stop pit 516. The material of the protectivelayer 452 is chosen to be resistant to the etchant that will be used toform the V-groove 512. For example, silicon dioxide is one suitablematerial. The silicon dioxide may be deposited by CVD or may be providedby thermal oxidation of the substrate surface. The silicon dioxide layershould be thick enough to serve as a mask during the etch-stop pitformation.

[0071] Following the application of the protective layer 452, anaperture definition layer 454 is deposited, at step 4010, over aselected portion of the protective layer 452, as shown in FIG. 45. Theaperture definition layer 454 is provided so that an aperture 457 may beprovided, as explained below, through which the V-groove 512 will beetched. The location of the aperture definition layer 454 is selected tocover that portion of the substrate surface at which the V-groove 512 isto be located.

[0072] Processing continues with the selective removal, at step 4020, ofa portion of the substrate 450 located within the etch-stop pit aperture451 to form an etch-stop pit 516 in the substrate 450, as depicted inFIG. 46. The etch-stop pit 516 may conveniently be formed by reactiveion etching, plasma etching, ion milling, or by any other directionalprocess. In addition, the etch-stop pit 516 may be formed by othermethods such as isotropic or anisotropic etching, so long as theetch-stop pit 516 attains the desired shape and depth.

[0073] Having created the etch-stop pit 516, the surfaces of theetch-stop pit 516 are covered, preferably conformally, with an etch-stoplayer 458, at step 4030, as illustrated in FIG. 47. The etch-stop layer458 may be conveniently provided by thermally oxidizing the substrate toprovide an etch-stop layer 458 comprising silicon dioxide. Anappropriate choice for the etch-stop layer 458 includes any materialthat is resistant to the etchant which will be used to create theV-groove 512. During the thermal oxidation step 4030, the previouslydeposited silicon dioxide protective layer 452 increases in thicknessand surrounds the perimeter of the aperture definition layer 454, asillustrated in FIG. 47.

[0074] With the etch-stop layer 458 in place, processing continues byremoving, at step 4040, the aperture definition layer 454 to provide aV-groove aperture 455 in the protective layer 452, as shown in FIG. 48.A sufficient thickness of the protective layer 452 is removed, at step4050, to expose the surface of the substrate 450 disposed below theaperture definition layer 454 so that the V-groove aperture 455communicates with the surface of the substrate 450. A portion of theprotective layer 452 and the etch-stop layer 458 remain on the surfaceswhere the V-groove 512 will not be formed, as illustrated in FIG. 49. Asuitable process for removing a thickness of the protective layer 452 isa short duration, wet or dry, oxide etch.

[0075] Next, as shown in FIG. 50, the portion substrate 450 accessiblethrough the V-groove aperture 455 is selectively removed, at step 4060,to form the V-groove 512, as illustrated in FIG. 50. Appropriateprocesses for the formation of the V-groove 512 include anisotropicetching with EDP or TMAH. KOH may also be used; however, since KOH canattack the protective layer 452 and etch-stop layer 458, KOH should onlybe used if the protective layer 452 and etch-stop layer 458 aresufficiently thick so as not to be completely removed by the KOH. As afinal optional step, the remaining portions of the protective layer 452and etch-stop layer 458 may be removed at step 4070, to yield the deviceillustrated in FIG. 51.

[0076] Referring now to FIGS. 41 and 52-58, another method in accordancewith the present invention is illustrated for creating the device shownin FIGS. 43 and 44. As illustrated in FIG. 52, a substrate 520 made from<100>-oriented Si is provided. The processing of the substrate 520begins at step 4100 of FIG. 41 by providing a first protective layer 522on a first surface of the substrate 520 to cover that portion of thesubstrate 520 in which neither the etch-stop pit 586 nor the V-groove582 is to be provided.

[0077] The first protective layer 522 may be deposited over the entiresurface of the substrate 520. Thereafter, portions of the firstprotective layer 522 may be removed to expose the surface of thesubstrate 520 at the selected areas for the etch-stop pit 586 and theV-groove 582. The material of the first protective layer 522 is chosento be resistant to the etchant that will be used to form the V-groove582. For example, silicon nitride is one suitable material. The siliconnitride may be deposited by CVD.

[0078] Following the application of the first protective layer 522, asecond protection layer 524 is deposited, at step 4110, over a selectedportion of the first protective layer 522 and the substrate surfacewhere the V-groove 582 is to be formed, as shown in FIG. 52. The secondprotection layer 524 includes an aperture 521 through which theetch-stop pit 586 may be formed.

[0079] Processing continues with the selective removal, at step 4120, ofa portion of the substrate 520 located within the etch-stop pit aperture521 to form an etch-stop pit 586 in the substrate 520, as depicted inFIG. 53. The etch-stop pit 586 may conveniently be formed by reactiveion etching, plasma etching, ion milling, or by any other directionalprocess. In addition, the etch-stop pit 586 may be formed by othermethods such as isotropic or anisotropic etching, so long as theetch-stop pit 586 attains the desired shape and depth.

[0080] Having created the etch-stop pit 586, the surfaces of theetch-stop pit 586 and second protective layer 524 are covered,preferably conformally, with an etch-stop layer 528, at step 4130, asillustrated in FIG. 54. The etch-stop layer 528 may be convenientlyprovided by CVD. An appropriate choice for the etch-stop layer 528includes any material that is resistant to the etchant which will beused to create the V-groove 582, such as silicon nitride.

[0081] With the etch-stop layer 528 in place, processing continues byremoving, at step 4140, the portion of the etch-stop layer 528 disposedon the upper surface 541 of second protective layer 524. The portion ofthe etch-stop layer 528 disposed within the etch-stop pit 586 isretained, as illustrated in FIG. 55. The removal step 4140 may beperformed by any suitable method such as planarization or polishing.Subsequently, at step 4150, a second protective layer 524 is removed, asshown in FIG. 56, to provide a V-groove aperture 525. A suitable methodfor removing the second protective layer 524 includes etching withdilute HF.

[0082] Next, as shown in FIG. 57, the portion substrate 520 accessiblethrough the V-groove aperture 525 is selectively removed, at step 4160,to form the V-groove 582, as illustrated in FIG. 50. Appropriateprocesses for the formation of the V-groove 582 include anisotropicetching with KOH. As a final optional step, the remaining portions ofthe first protective layer 522 and etch-stop layer 528 may be removed atstep 4170, to yield the device illustrated in FIG. 58.

[0083] Referring now to FIGS. 42 and 59-64, yet another method inaccordance with the present invention is illustrated for creating thedevice shown in FIGS. 43 and 44. As illustrated in FIG. 59, a substrate590 made from <100>-oriented Si is provided. The processing of thesubstrate 590 begins at step 4200 of FIG. 42 by providing protective anaperture definition layer 594 deposited over a selected portion of thesubstrate 590 , as shown in FIG. 59. The location of the aperturedefinition layer 594 is selected to cover that portion of the substratesurface at which the V-groove 632 is to be located. A suitable materialfor use as the aperture definition layer 524 is silicon nitride.

[0084] The processing of the substrate 590 continues, at step 4210, byproviding a photoresist layer 592 over the aperture definition layer 594and over the portions of the substrate 590 not covered by the aperturedefinition layer 524. Photoresist layer 592 is patterned, using methodsknown in the art, to provide an etch-stop pit aperture 591, asillustrated in FIG. 59. Processing continues with the selective removal,at step 4220, of a portion of the substrate 590 located within theetch-stop pit aperture 591 to form an etch-stop pit 636 in the substrate590 , as depicted in FIG. 60. The etch-stop pit 636 may conveniently beformed by a process which does not remove the aperture definition layer594. In addition, the etch-stop pit 636 may be formed by other methodssuch as isotropic or anisotropic etching, so long as the etch-stop pit636 attains the desired shape and depth.

[0085] Having created the etch-stop pit 636, the photoresist layer 592is removed, at step 4230. The surfaces of the etch-stop pit 636 andexposed surfaces of the substrate 590 are oxidized to form an etch-stoplayer 598, at step 4230, as illustrated in FIG. 61. With the etch-stoplayer 598 in place, processing continues by removing, at step 4240, theaperture definition layer 594 to provide an un-oxidized region 597 ofthe substrate 590 , as shown in FIG. 62.

[0086] Next, as shown in FIG. 63, the un-oxidized region 597 of thesubstrate 590 is selectively removed, at step 4250, to form the V-groove632, as illustrated in FIG. 63. Appropriate processes for the formationof the V-groove 632 include anisotropic etching with EDP or TMAH. KOHmay also be used; however, since KOH can attack oxide etch-stop layer598, KOH should only be used if the etch-stop layer 598 is sufficientlythick so as not to be completely removed by the KOH. As a final optionalstep, the remaining portions of the etch-stop layer 598 may be removedat step 4260, to yield the device illustrated in FIG. 64.

[0087] These and other advantages of the present invention will beapparent to those skilled in the art from the foregoing specification.Accordingly, it will be recognized by those skilled in the art thatchanges or modifications may be made to the above-described embodimentswithout departing from the broad inventive concepts of the invention.For example, a non-anisotropically etched feature may be formed adjacentan etch-stop pit. It should therefore be understood that this inventionis not limited to the particular embodiments described herein, but isintended to include all changes and modifications that are within thescope and spirit of the invention as set forth in the claims.

What is claimed is:
 1. An optical microbench, comprising substratehaving an etch-stop pit and an anisotropically etched feature disposedadjacent the etch-stop pit.
 2. The optical microbench according to claim1, wherein the anisotropically etched feature comprises a V-groove.
 3. Amethod for micromachining a substrate comprising the step of forming anetch-stop pit and the step of forming an anisotropically etched featureadjacent the etch-stop pit.
 4. The method according to claim 3comprising coating the surfaces of the etch-stop pit with an etch-stoplayer.